1. Field of the Invention
This invention relates to an Fe—Cr alloy structure and a manufacturing method for the structure, wherein a structure such as a member or a component for an electrical device, precision machining machine, automobile, building material, and the like, is formed of Fe—Cr alloy, a typical form of stainless steel, and wherein, in the event that a gap and/or seam is formed by formation or assembling, the corrosion resistance and paint film adhesion to the gap/seam is improved.
Furthermore, this invention relates to a stainless steel suitable for members disposed around the underside of an automobile (which will be referred to as “automobile underside members”, in particular to a zinc-containing paint-applied Fe—Cr alloy structure, and a manufacturing method for the structure.
Moreover, the invention relates to a ferritic stainless steel which is suitably employed in containers or piping members, for organic fuel such as gasoline, methanol, or the like, and particularly relates to zinc-containing paint-applied ferritic stainless steel for a fuel tank or fuel tank peripheral members of an automobile, wherein zinc-containing paint has been applied to the entirety of stainless steel members for fuel tanks, or fuel tank peripheral members such as fuel pipes, tank bands, or the like, of an automobile, or has been applied to part thereof principally to improve the corrosion resistance of the gap/seam, and relates to a manufacturing method for the zinc-containing paint-applied ferritic stainless steel.
2. Description of the Related Art
Fe—Cr alloy, a typical form of stainless steel, (which is referred to as “Fe—Cr alloy” hereinafter) has been widely used for various purposes, wherein resistance to various types of corrosion is required, due to excellent corrosion-resistance, and in particular, in various purposes wherein the advantages of appearance, design, and being maintenance-free, are taken in full. However, it has been found that in practical-use environments, the corrosion resistance is not sufficient, and in particular, corrosion readily occurs at gaps/seams, welds, connected portions to dissimilar metal or other materials, or the like, (which is referred to as “gap portions” and “gaps” hereinafter), and the corrosion progresses.
For example, the corrosion resistance of a gap between plates formed by welding two plates, a gap between a bolt (metal fitting) and a base member, or the like, has been insufficient. Such gaps are formed in most structures, not to mention automobiles or building materials. Conventionally, the kind of Fe—Cr alloy is selected based upon the required corrosion resistance of the portion wherein highest corrosion resistance is required, i.e., the gaps, and accordingly, the selected alloy might have excessively high quality for portions other than gaps, or for use in a non-corrosive environment.
To improve the corrosion resistance of stainless steel, in particular, at gaps, an arrangement wherein metal with ionization tendencies higher than stainless steel is inserted into gaps, an arrangement wherein metal foil with ionization tendencies higher than stainless steel is placed over portions which are to be gaps after manufacturing, and an arrangement wherein paint containing metal with ionization tendencies higher than stainless steel is applied to the portions, have been proposed in Japanese Unexamined Patent Publication No. 11-79285.
However, with the arrangement wherein metal foil is placed over the portions, the number of manufacturing steps increases, and also the number of members increase. Hence, the arrangement is impractical, whereas the arrangement wherein the paint is applied to the portions can be made in a simple manner. Accordingly, we performed experiments wherein commercially-available zinc-rich paint (paint containing metal zinc powder) was applied to the gap and therearound of a testing sample for evaluation according to the Japanese Unexamined Patent Application Publication No. 11-79285, and the improvement of the corrosion resistance was observed in an experiment wherein the testing sample was exposed to an experimental corrosive environment. We discovered that in the event that the paint was applied to an actual structure, following which the structure was further processed, or the structure was transported, rust occurring in a conventional arrangement was observed in this arrangement, and the improvement of the corrosion resistance was insufficient.
Furthermore, a new discovery was made, that even in the event that zinc-rich paint is applied to a gap after processing, upon zinc being dissolved from the paint film due to extended use, the paint film becomes porous. This leads to further occurrence of corrosion due to penetration of salt particles, rain water, or the like, and also leads to paint film peeling, which damages the appearance.
Of structural members, in particular, automobile structural members are exposed to severe environments. Of the automobile structural members, strict corrosion resistance of gaps is required for underside members, and a material for those has been desired. Principal properties required for the automobile underside members will be described below:    1) The automobile underside members are welded to a car body. Thus, toughness is required for the welded portions. In particular, the properties of welded heat-affected zone (HAZ) depend upon the properties of the steel itself. Hence, it is important to improve the properties of the HAZ.    2) With the automobile underside members, gaps are formed due to forming or assembling. Adhesion or penetration of water, mud, sea-salt particles, road salt, or the like occurs at the gaps in practical-use environments. Accordingly, there is the need to improve the corrosion resistance, in particular, from the perspective of gap corrosion in the salty environments.    3) High tensile strength (TS) around 450 to 650 MPa is required for structural members.
As a result, conventionally, a material excellent in at least welded portion toughness, corrosion resistance (in particular, gap corrosion resistance), and strength (in particular, welded portion strength) has been required for automobile underside members. For example, in the event that the high-tensile steel of the ordinary steel is subjected to electrodeposition coating for painting corrosion-resistant paint, or is subjected to plating, to manufacture the automobile underside members, there is the need to perform corrosion-resistant processes under sufficient quality control so that rust never occurs due to painting, plating, or the like. Accordingly, large-size equipment is required for performing corrosion-resistant processing to prevent occurrence of uneven portions due to painting, plating, or the like, at end portions, scratches, welded portions, or the like, after manufacturing. This causes productivity decreases, and consequently, increases in costs for painting.
Accordingly, high strength stainless steel wherein painting or plating processing can be simplified, and also corrosion-resistant processes can be simplified with excellent corrosion resistance attracts attention as a material of automobile underside members.
For example, Cr-containing stainless steel wherein welded portion strength and toughness are improved has been studied in Japanese Unexamined Patent Publication No. 55-21566. Furthermore, to improve the corrosion resistance, various methods for improving properties by adjusting the chemical composition of steel have been studied in Japanese Unexamined Patent Application Publication No. 2002-20844.
However, arrangements in the conventional art have preconditions of improving the corrosion resistance of steel without painting. Accordingly, there is the need to add considerably a great quantity of Cr to the steel for maintaining corrosion resistance in salty environments. Furthermore, in the event that martensite structure is employed in the steel for securing the strength and toughness as automobile underside members, there is the need to add expensive alloy elements such as Ni, Cu, or the like, which are austenite stabilizing elements, to the steel.
Conventionally, ternes steel sheets (Pb—Sn) wherein the surface of mild steel plates have been subjected to plating including Pb have been widely used for automobile fuel tanks and fuel tank peripheral members (fuel pipes and so forth). However, in recent years, use of materials containing Pb has been severely restricted because of increasing environmental problems. Therefore, development of alternative materials for ternes steel sheets is being undertaken.
For example, a steel plate subjected to Al—Si alloy plating as unleaded plating, and further subjected to conversion treatment for improving salt-induced corrosion resistance, has been proposed in Japanese Unexamined Patent Publication No. 2002-146553. However, there are problems of weldability and deterioration of corrosion resistance over time. Thus, the steels have not been widely used. Furthermore, in the event that large-size equipment is prepared for obtaining the steel plates, the costs increase, leading to low productivity, and consequently, the arrangement cannot sufficiently meet the demands for mass production.
Furthermore, stainless steel for fuel tanks, which secures resistance weldability, press-workability due to lubrication hardening, and corrosion resistance by zinc or a lubricant film containing zinc being painted on the plate prior to processing, has been proposed in Japanese Unexamined Patent Application Publication No. 2002-146557.
However, the steel plate coated with zinc-containing lubrication film is subjected to resistance welding, carbon is mixed from the resin components of the film into the welded portion, and consequently, deterioration of corrosion resistance could occur due to the sensitivity thereof. Furthermore, in the event that the steel plate having a zinc-containing lubrication film is subjected to press-forming, peeling powder markedly occurs in pressing as compared with a lubrication film containing no zinc, leading to difficulties in maintenance of molds.
Furthermore, use of austenitic stainless steel of which a typical form is SUS304 stipulated by JISG4305 (cold-rolled stainless steel plate and steel strip) is being undertaken as steel which can be use without lining processing; However, there is the problem of stress corrosion cracking (SCC) with regard to use in fuel tanks, and consequently, the arrangement has not been put to practical use, either.
Moreover, use of synthetic resin having multilayer structures for fuel tanks is being undertaken. However, penetration of minute amounts of fuel from the wall face of the fuel tank formed of resin is unavoidable. Furthermore, there is a fundamental problem of fuel evaporation. Also, there is an inherent limit in application of synthetic resin in practical use from the points of restriction of fuel evaporation and restriction for recycling.
On the other hand, ferritic stainless steel, of which typical forms are SUS430 and SUS436L stipulated by JISG4305 has low sensitivity with regard to stress corrosion cracking as compared with the aforementioned austenitic stainless steel. Furthermore, the content of expensive Ni is low, so the ferritic stainless steel has a cost advantage. However, in the event of employing the ferritic stainless steel in fuel tanks or fuel pipes, principally, there is a problem of corrosion resistance of the outer face thereof against salt-induced corrosion being insufficient. Therefore, there is the need to add a great quantity of alloy elements such as Cr, Mo, or the like, into the ferritic stainless steel. However, deterioration of workability occurs accompanying high alloying, so great tube expansion or bending cannot be performed for fuel pipes, for example, leading to limits in forming shapes.